Morphological analysis of the early stages of oligodendrocyte development in the vertebrate central nervous system.

Oligodendrocytes are responsible for myelin formation in the vertebrate central nervous system. While in vitro analysis have provided critical information on the cellular properties of oligodendrocyte precursors, they provide limited information on the morphological development of these cells in the intact CNS. Recent studies have begun to provide insights into when and where oligodendrocyte precursors arise in the neural tube. In the chick CNS, the monoclonal antibody O4 selectively labels oligodendrocyte lineage cells both in vitro and in vivo and here we discuss the characteristics of O4+ oligodendrocyte precursors during development of the chick CNS. The earliest oligodendrocytes initially develop in restricted locations in the CNS. In the spinal cord, for example, oligodendrocyte precursors arise in the ventral ventricular zone dorsal to the floor plate. These early oligodendrocyte precursors are integral components of the lining of the central canal and have an embryonic neuroepithelial cell morphology, suggesting that commitment to the oligodendrocyte lineage occurs in the ventricular zone. With maturation these early oligodendrocyte precursors lose their ventricular connection, adopt a uni- or-bipolar morphology, and migrate throughout the CNS. When these cells reach presumptive white matter, they stop migrating, become multiprocessed, and differentiate into immature oligodendrocytes. The maturation of these newly formed oligodendrocytes results in the upregulation of expression of a variety of myelin specific genes such as MBP and PLP and the subsequent elaboration of the myelin organelle. In the developing optic nerve, the onset of myelination occurs several days after oligodendrocyte precursors populate the nerve, suggesting that additional signals are required to induce myelin formation.